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In a method for suppressing a noise by the spectrum subtraction method, it
is possible to improve the noise suppression capability by simultaneously
obtaining a frequency resolution required for the noise estimation
spectrum and a temporal resolution required for the noise suppression
spectrum. The signal length of an observation signal cut out for
analyzing the spectrum of the observation signal used for estimation
calculation of the noise spectrum is set longer than the signal length of
an observation signal cut out for analyzing the spectrum of the
observation signal as a value to be subtracted for performing subtraction
with the noise spectrum.

1. A noise suppressing method for obtaining, from an observation signal in
which noise is superimposed on a sound, a sound in which the noise is
suppressed, comprising:extracting a second observation signal from the
observation signal;analyzing a spectrum of the second observation
signal;estimation-calculating a noise spectrum on the basis of the
spectrum of the second observation signal;extracting a first observation
signal from the observation signal;analyzing a spectrum of the first
observation signal;subtracting the noise spectrum from the spectrum of
the first observation signal; andconverting the sound spectrum into a
signal in the time domain,wherein a signal length of the second
observation signal is longer than that of the first observation signal.

2. A noise suppressing method, comprising:extracting a part of an
observation signal that progresses with time and in which noise is
superimposed on a sound, every time a prescribed interval of time with
which the observation signal progresses elapses, in a first signal length
that is longer than or equal to the prescribed time interval;analyzing,
as a first spectrum, a spectrum of the observation signal that is
extracted in the first signal length;extracting apart of the observation
signal every time the prescribed time interval or a proper time elapses
in a second signal length that is longer than the first signal length in
such a manner that its head coincides with a head of the observation
signal that is extracted in the first signal length;analyzing, as a
second spectrum, a spectrum of the observation signal that is extracted
in the second signal length;estimation-calculating a spectrum of noise
included in the observation signal on the basis of the second
spectrum;subtracting the noise spectrum from the first spectrum every
time the prescribed time interval elapses, to calculate a
noise-suppressed sound spectrum;converting the calculated sound spectrum
into a signal in the time domain every time the prescribed time interval
elapses; andobtaining a continuous noise-suppressed sound by connecting
the converted time-domain signals to each other.

3. The noise suppressing method according to claim 2,
comprising:smoothing-processing the second spectrum;
andestimation-calculating a noise spectrum on the basis of a
smoothing-processed second spectrum.

4. The noise suppressing method according to claim 2, wherein the
subtracting process is executed after the estimated noise spectrum is
subjected to smoothing processing.

5. The noise suppressing method according to claim 2, wherein the
estimation-calculating process includes:smoothing-processing the second
spectrum;comparing a smoothing-processed second spectrum with the second
spectrum that is not smoothing-processed;choosing larger values at
respective frequency points in the comparing process, to eliminate a dip
in the second spectrum; andestimation-calculating a noise spectrum on the
basis of a dip-eliminated second spectrum.

6. The noise suppressing method according to claim 2, wherein the
subtracting process includes:smoothing-processing the estimated noise
spectrum;comparing a smoothing-processed noise spectrum with the noise
spectrum that has is not smoothing-processed;choosing larger values at
respective frequency points in the comparing process, to eliminate a dip
in the noise spectrum; andsubtracting a dip-eliminated noise spectrum
from the first spectrum.

7. The noise suppressing method according to claim 2, comprising:adding a
zero signal having a prescribed length after an end of the observation
signal that is extracted in the first signal length so that a signal
length of the observation signal to be used for the analysis of the first
spectrum is made equal to the second signal length;analyzing, as a first
spectrum, a spectrum of the observation signal to which the zero signal
is added;subtracting the noise spectrum from the analyzed first
spectrum;converting a sound spectrum that is obtained by the subtracting
process into a signal in the time domain;removing a signal having the
same length as the added zero signal located after an end of the
time-domain signal, to return a signal length of the time-domain signal
to the first signal length; andconnecting the time-domain signals to each
other whose signal length is returned to the first signal length.

8. The noise suppressing method according to claim 2, wherein the
prescribed time interval is a half of the first signal length.

9. The noise suppressing method according to claim 8, wherein the
time-domain signal is a signal that is obtained in the first signal
length every time the prescribed time interval elapses, and wherein the
time-domain signal is multiplied by a triangular window and the
time-domain signals that are multiplied by the triangular window are
added to each other sequentially and thereby connected to each other.

10. A noise suppressing apparatus for obtaining a noise-suppressed sound
from an observation signal in which noise is superimposed on a sound,
comprising:a first spectrum analyzing section which analyzes a spectrum
of an observation signal having a first signal length;a second spectrum
analyzing section which analyzes a spectrum of an observation signal
having a second signal length;a noise spectrum estimation-calculating
section which estimation-calculates a noise spectrum on the basis of the
observation signal spectrum that is analyzed by the second spectrum
analyzing section;a subtracting section which subtracts the noise
spectrum from the spectrum that is analyzed by the first spectrum
analyzing section, to calculate a noise-suppressed sound spectrum; anda
conversion-into-time-domain section which converts the calculated sound
spectrum into a signal in the time domain,wherein a signal length of the
second observation signal is longer than that of the first observation
signal.

11. A noise suppressing apparatus, comprising:a first signal extracting
section which extracts a part of an observation signal that progresses
with time and in which noise is superimposed on a sound, every time a
prescribed interval of time with which the observation signal progresses
elapses, in a first signal length that is longer than or equal to the
prescribed time interval;a first spectrum analyzing section which
analyzes, as a first spectrum, a spectrum of the observation signal that
is extracted by the first signal extracting section;a second extracting
section which extracts a part of the observation signal every time the
prescribed time interval or a proper time elapses in a second signal
length that is longer than the first signal length in such a manner that
its head coincides with a head of the observation signal that is
extracted in the first signal length;a second spectrum analyzing section
which analyzes, as a second spectrum, a spectrum of the observation
signal that is extracted by the second signal extracting section;a noise
spectrum estimation-calculating section which estimation-calculates a
spectrum of noise included in the observation signal on the basis of the
second spectrum;a subtracting section which subtracts the noise spectrum
from the first spectrum every time the prescribed time interval elapses,
to calculate a noise-suppressed sound spectrum;a
conversion-into-time-domain section which converts the calculated sound
spectrum into a signal in the time domain every time the prescribed time
interval elapses; andan output combining section which obtains a
continuous noise-suppressed sound by connecting the converted time-domain
signals to each other.

12. A noise suppressing method for obtaining, from an observation signal
in which noise is superimposed on a sound, a sound in which the noise is
suppressed, comprising:analyzing a spectrum of the observation
signal;smoothing-processing the observation signal spectrum;comparing the
smoothing-processed observation signal spectrum with the observation
signal spectrum that is not smoothing-processed;choosing larger values at
respective frequency points in the comparing process, to eliminate a dip
from the observation signal spectrum;estimation-calculating a noise
spectrum on the basis of a dip-eliminated observation signal
spectrum;subtracting the noise spectrum from the observation signal
spectrum, to calculate a sound spectrum in which the noise is suppressed;
andconverting the sound spectrum into a signal in the time domain.

13. A noise suppressing method for obtaining, from an observation signal
in which noise is superimposed on a sound, a sound in which the noise is
suppressed, comprising:analyzing a spectrum of the observation
signal;estimation-calculating a noise spectrum on the basis of the
observation signal spectrum;smoothing-processing the estimated noise
spectrum;comparing a smoothing-processed noise spectrum with the noise
spectrum that is not smoothing-processed;choosing larger values at
respective frequency points in the comparing process, to eliminate a dip
from the noise spectrum;subtracting the noise spectrum from the
observation signal spectrum, to calculate a sound spectrum in which the
noise is suppressed; andconverting the sound spectrum into a signal in
the time domain.

Description

TECHNICAL FIELD

[0001]The present invention relates to a method and apparatus for
suppressing noise by a spectrum subtraction method, which are increased
in noise suppression performance.

BACKGROUND ART

[0002]The spectrum subtraction method is one of various techniques for
suppressing noise that is included in a sound. The spectrum subtraction
method determines a spectrum of an observation signal in which noise is
superimposed on a sound (hereinafter referred to as "observation signal
spectrum"), estimates a spectrum of noise (hereinafter referred to as
"noise spectrum") from the observation signal spectrum, and obtains a
spectrum of a noise-suppressed sound (hereinafter referred to as "sound
spectrum") by subtracting the noise spectrum from the observation signal
spectrum. The spectrum subtraction method then produces a
noise-suppressed sound by converting the sound spectrum into a signal in
the time domain.

[0003]Examples of conventional techniques that include the spectrum
subtraction technique are described in the following patent documents:

[0004][Patent document 1] JP-A-11-3094

[0005][Patent document 2] JP-A-2002-14694

[0006][Patent document 3] JP-A-2003-223186

[0007]In the conventional spectrum subtraction method, a common
observation signal spectrum is used as an observation signal spectrum
used for estimation-calculating a noise spectrum (hereinafter referred to
as "noise estimation spectrum") and as an observation signal spectrum as
a minuend from which to subtract the noise spectrum (hereinafter referred
to as "noise suppression spectrum").

DISCLOSURE OF THE INVENTION

Problems to Be Solved By the Invention

[0008]Noise as a subject of suppression of the spectrum subtraction method
is noise that does not vary much in time, such as stationary noise.
Therefore, as long as the noise estimation spectrum is concerned, the
frequency resolution is more important than the time resolution. In
contrast, a sound as a subject of extraction of the spectrum subtraction
method is a signal that varies much in time. Therefore, as long as the
noise suppression spectrum is concerned, it is important that the time
resolution be high. However, since a common observation signal spectrum
is used as a noise estimation spectrum and as a noise suppression
spectrum, the conventional spectrum subtraction method cannot satisfy
both of frequency resolution that is necessary for the noise estimation
spectrum and time resolution that is necessary for the noise suppression
spectrum. As such, the conventional spectrum subtraction method is not
sufficiently high in noise suppression performance.

[0009]The present invention has been made in view of the above points, and
an object of the invention is therefore to provide a noise suppression
method and a noise suppression apparatus which satisfy both of frequency
resolution that is necessary for a noise estimation spectrum and time
resolution that is necessary for a noise suppression spectrum and hence
is increased in noise suppression performance.

Means for Solving the Problems

[0010]A noise suppressing method according to the invention for obtaining,
from an observation signal in which noise is superimposed on a sound, a
sound in which the noise is suppressed comprises the steps of extracting
a first observation signal from the observation signal; analyzing a
spectrum of the first observation signal; estimation-calculating a noise
spectrum on the basis of the spectrum of the first observation signal;
extracting a second observation signal from the observation signal;
analyzing a spectrum of the second observation signal; subtracting the
noise spectrum from the spectrum of the second observation signal; and
converting a sound spectrum into a signal in the time domain, wherein a
signal length (time window length) of the first observation signal is
longer than that of the second observation signal.

[0011]This noise suppressing method according to the invention can
increase the frequency resolution that is necessary for a noise
estimation spectrum, because the signal length of an observation signal
that is extracted to analyze its spectrum to be used for
estimation-calculating a noise spectrum is set relatively long.
Furthermore, the noise suppressing method can increase the time
resolution that is necessary for a noise suppression spectrum, because
the signal length of an observation signal that is extracted to analyze
its spectrum as a minuend from which to subtract a noise spectrum is set
relatively short. As a result, both of frequency resolution that is
necessary for a noise estimation spectrum and time resolution that is
necessary for a noise suppression spectrum can be satisfied and hence the
noise suppression performance can be increased.

[0012]A noise suppressing method according to the invention, which is a
more specific version, comprises the steps of extracting a part an
observation signal that progresses with time and in which noise is
superimposed on a sound, every time a prescribed interval of time with
which the observation signal progresses elapses, in a first signal length
that is longer than or equal to the prescribed time interval; analyzing,
as a first spectrum, a spectrum of the observation signal that has been
extracted in the first signal length; extracting a part of the
observation signal every time the prescribed time interval or a proper
time elapses in a second signal length that is longer than the first
signal length in such a manner that its head coincides with a head of the
observation signal that is extracted in the first signal length;
analyzing, as a second spectrum, a spectrum of the observation signal
that has been extracted in the second signal length;
estimation-calculating a spectrum of noise included in the observation
signal on the basis of the second spectrum; subtracting the noise
spectrum from the first spectrum every time the prescribed time interval
elapses, to calculate a noise-suppressed sound spectrum; converting the
calculated sound spectrum into a signal in the time domain every time the
prescribed time interval elapses; and obtaining a continuous
noise-suppressed sound by connecting the converted time-domain signals to
each other.

[0013]This noise suppressing method according to the invention comprises
the steps of smoothing-processing the second spectrum, and
estimation-calculating a noise spectrum on the basis of a
smoothing-processed second spectrum. Alternatively, the subtracting step
is executed after the estimated noise spectrum is subjected to smoothing
processing. By virtue of the smoothing processing, the substantial
frequency resolution of the noise spectrum is made equal to (or close to)
that of the first spectrum. The above steps of calculating a noise
estimation spectrum at a high resolution using long-term data and
smoothing it increase the accuracy (effectiveness) of each subtraction
result (sound spectrum data).

[0014]In the above noise suppressing method according to the invention,
the estimation-calculating step comprises the substeps of
smoothing-processing the second spectrum; comparing a smoothing-processed
second spectrum with the second spectrum that has not been
smoothing-processed; choosing larger values at respective frequency
points in the comparing substep, to eliminate dips in the second
spectrum; and estimation-calculating a noise spectrum on the basis of a
dip-eliminated second spectrum. Alternatively, the subtracting step
comprises the substeps of smoothing-processing the estimated noise
spectrum; comparing a smoothing-processed noise spectrum with the noise
spectrum that has not been smoothing-processed; choosing larger values at
respective frequency points in the comparing substep, to eliminate dips
in the noise spectrum; and subtracting a dip-eliminated noise spectrum
from the first spectrum. When a spectrum of an observation signal to be
used for estimation-calculating a noise spectrum is analyzed, large dips
occur in a resulting spectrum and may result in processing noise (i.e.,
noise that is newly generated by signal processing; musical noise).
Occurrence of processing noise can be suppressed by
estimation-calculating a noise spectrum after eliminating dips from the
second spectrum or subtracting a noise spectrum from the first spectrum
after eliminating dips from the noise spectrum. The technique of
eliminating dips from a noise spectrum or an observation signal spectrum
to be used for estimation-calculating a noise spectrum can be applied to
not only the case that the signal length of an observation signal that is
extracted to analyze an observation signal spectrum to be used for
estimation-calculating a noise spectrum is set longer than the signal
length of an observation signal that is extracted to analyze an
observation signal spectrum as a minuend from which to subtract a noise
spectrum, but also a case that the two kinds of signal length are set
identical.

[0015]The above noise suppressing method according to the invention
comprises the steps of adding a zero signal having a prescribed length
after an end of the observation signal that has been extracted in the
first signal length so that a signal length of the observation signal to
be used for the analysis of the first spectrum is made equal to the
second signal length; analyzing, as a first spectrum, a spectrum of the
observation signal to which the zero signal is added; subtracting the
noise spectrum from the analyzed first spectrum; converting a sound
spectrum that has been obtained by the subtracting step into a signal in
the time domain; removing a signal having the same length as the added
zero signal located after an end of the time-domain signal, to return a
signal length of the time-domain signal to the first signal length; and
connecting the time-domain signals to each other whose signal length is
returned to the first signal length.

[0016]In the above noise suppressing method according to the invention,
the prescribed time interval may be, for example, a half of the first
signal length. In this case, the noise suppressing method may be such
that the time-domain signal is a signal that is obtained in the first
signal length every time the prescribed time interval elapses, and that
the time-domain signal is multiplied by a triangular window and the
time-domain signals that have been multiplied by the triangular window
are added to each other sequentially and thereby connected to each other.

[0017]A noise suppressing apparatus according to the invention for
obtaining a noise-suppressed sound from an observation signal in which
noise is superimposed on a sound comprises a first signal extracting
section for extracting a part an observation signal that progresses with
time and in which noise is superimposed on a sound, every time a
prescribed interval of time with which the observation signal progresses
elapses, in a first signal length that is longer than or equal to the
prescribed time interval; a first spectrum analyzing section for
analyzing, as a first spectrum, a spectrum of the observation signal that
has been extracted by the first signal extracting section; a second
extracting section for extracting a part of the observation signal every
time the prescribed time interval or a proper time elapses in a second
signal length that is longer than the first signal length in such a
manner that its head coincides with a head of the observation signal that
is extracted in the first signal length; a second spectrum analyzing
section for analyzing, as a second spectrum, a spectrum of the
observation signal that has been extracted by the second signal
extracting section; a noise spectrum estimation-calculating section for
estimation-calculating a spectrum of noise included in the observation
signal on the basis of the second spectrum; a subtracting section for
subtracting the noise spectrum from the first spectrum every time the
prescribed time interval elapses, to calculate a noise-suppressed sound
spectrum; a conversion-into-time-domain section for converting the
calculated sound spectrum into a signal in the time domain every time the
prescribed time interval elapses; and an output combining section for
obtaining a continuous noise-suppressed sound by connecting the converted
time-domain signals to each other

[0018]A noise suppressing apparatus according to the invention, which is a
more specific version, comprises a first signal extracting section for
extracting a part an observation signal that progresses with time and in
which noise is superimposed on a sound, every time a prescribed interval
of time with which the observation signal progresses elapses, in a first
signal length that is longer than or equal to the prescribed time
interval; a first spectrum analyzing section for analyzing, as a first
spectrum, a spectrum of the observation signal that has been extracted by
the first signal extracting section; a second extracting section for
extracting a part of the observation signal every time the prescribed
time interval or a proper time elapses in a second signal length that is
longer than the first signal length in such a manner that its head
coincides with a head of the observation signal that is extracted in the
first signal length; a second spectrum analyzing section for analyzing,
as a second spectrum, a spectrum of the observation signal that has been
extracted by the second signal extracting section; a noise spectrum
estimation-calculating section for estimation-calculating a spectrum of
noise included in the observation signal on the basis of the second
spectrum; a subtracting section for subtracting the noise spectrum from
the first spectrum every time the prescribed time interval elapses, to
calculate a noise-suppressed sound spectrum; a
conversion-into-time-domain section for converting the calculated sound
spectrum into a signal in the time domain every time the prescribed time
interval elapses; and an output combining section for obtaining a
continuous noise-suppressed sound by connecting the converted time-domain
signals to each other.

[0019]Another noise suppressing method according to the invention for
obtaining, from an observation signal in which noise is superimposed on a
sound, a sound in which the noise is suppressed comprises the steps of
analyzing a spectrum of the observation signal; smoothing-processing the
observation signal spectrum; comparing the smoothing-processed
observation signal spectrum with the observation signal spectrum that has
not been smoothing-processed; choosing larger values at respective
frequency points in the comparing step, to eliminate dips from the
observation signal spectrum; estimation-calculating a noise spectrum on
the basis of a dip-eliminated observation signal spectrum; subtracting
the noise spectrum from the observation signal spectrum, to calculate a
sound spectrum in which the noise is suppressed; and converting the sound
spectrum into a signal in the time domain.

[0020]A further noise suppressing method according to the invention for
obtaining, from an observation signal in which noise is superimposed on a
sound, a sound in which the noise is suppressed comprises the steps of
analyzing a spectrum of the observation signal; estimation-calculating a
noise spectrum on the basis of the observation signal spectrum;
smoothing-processing the estimated noise spectrum; comparing a
smoothing-processed noise spectrum with the noise spectrum that has not
been smoothing-processed; choosing larger values at respective frequency
points in the comparing steps to eliminate dips from the noise spectrum;
subtracting the noise spectrum from the observation signal spectrum, to
calculate a sound spectrum in which the noise is suppressed; and
converting the sound spectrum into a signal in the time domain.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a flowchart outlining the procedure of a noise suppressing
process which utilizes a noise suppression method according to the
invention.

[0022]FIG. 2 is an explanatory diagram of the noise suppressing process.

[0023]FIG. 3 shows functional blocks of an embodiment of a noise
suppressing apparatus for executing the noise suppressing process of FIG.
1.

[0024]FIG. 4 is a spectrum diagram showing the operation of a dip
eliminating section 22 shown in FIG. 2.

[0026]FIG. 6 is a waveform diagram showing differences between output
waveforms that were obtained when stationary noise was input in a
conventional spectrum subtraction method and the spectrum subtraction
method according to the invention.

[0027]FIG. 7 is a waveform diagram of a case that a sound with noise is
input to the noise suppressing apparatus according to the invention.

[0038]Embodiments of the present invention will be hereinafter described.
FIG. 1 outlines the procedure of a noise suppressing process which
utilizes a noise suppression method according to the invention. FIG. 2 is
an explanatory diagram of the noise suppressing process. In FIG. 1, an
observation signal x.sub.0(n) (n=0, 1, 2, . . . ) as a subject of noise
suppression is a sequence of samples of an audio signal that is produced
by a microphone or the like and include noise (e.g., an audio signal
received through a telephone communication or a signal that is input for
speech recognition) and is an audio signal with noise of a target sound
of a speaker that is mixed with stationary noise such as background
noise. The observation signal x.sub.0(n) is subjected to frame extracting
(signal extracting) in different frame lengths (signal lengths, time
window lengths) for analysis of a noise suppression spectrum and for
analysis of a noise suppression spectrum (S1 and S2). That is, frames for
analysis of a noise suppression spectrum are extracted from the
observation signal x.sub.0(n) in a relatively short frame length T1 (S1;
the relatively short frame length T1 and frames that are extracted from
the observation signal x.sub.0(n) in this frame length will be
hereinafter referred to as "noise suppression frame length" and "noise
suppression frames," respectively) and frames for analysis of a noise
estimation spectrum are extracted from the observation signal x.sub.0(n)
in a relatively great length T2 (S2; the relatively great frame length T2
and frames that are extracted from the observation signal x.sub.0(n) in
this frame length will be hereinafter referred to as "noise estimation
frame length" and "noise estimation frames," respectively). A noise
suppression frame and a noise estimation frame are extracted from the
observation signal (S1 and S2) repeatedly, that is, every time a half of
the noise suppression frame length T1 elapses, in such a manner that the
heads of the noise suppression frame and the noise estimation frame are
timed with each other (i.e., observation signal samples (latest samples)
of the same time point are located at the heads of the two frames). Zero
data having a prescribed length (i.e., sample data whose signal values
are zero, a zero signal) are added to each extracted noise suppression
frame immediately after its end (its last sample), whereby the frame
length is made equal to the noise estimation frame length T2 formally (in
a simulated manner) (S3). This processing is performed because to
subtract a noise spectrum from a noise suppression spectrum it is
necessary that the numbers of data (the numbers of frequency points) of
the two spectra be the same. That is, the number of data of the noise
spectrum is the same as that of a noise estimation spectrum, and to
equalize the number of data of the noise suppression spectrum to that of
the noise estimation spectrum it is necessary to equalize the numbers of
data (the numbers of samples) of the noise suppression spectrum and the
noise estimation spectrum in the time domain before conversion into data
in the frequency domain. Where a sound as a subject of extraction is a
voice of a speaker, the noise suppression frame length T1 can be set at
20 to 32 ms, for example. Where noise as a subject of suppression is room
air-conditioning noise, the noise estimation frame length T2 can be set
about eight times longer than the noise suppression frame length T1
(e.g., 256 ms).

[0039]In FIG. 2, "(a) Process before noise suppression" is the
above-described steps S1-S3. More specifically, every time M/2 samples of
an observation signal is newly input (every time T1/2 elapses), latest M
samples of the observation signal are extracted as a noise suppression
frame (i.e., noise suppression frames are extracted with an overlap of
M/2 samples) and latest N samples (N>M; in FIG. 2, N is set equal to
8M) of the observation signal are extracted as a noise estimation frame.
Zero data of (N-M) samples are added after the end of each noise
suppression frame, whereby the frame length of each noise suppression
frame is made equal to the noise estimation frame length T2 formally.

[0040]Referring to FIG. 1, every time the data of a noise suppression
frame are extracted (i.e., for each time interval corresponding to M/2
samples of the observation signal), the data of the noise suppression
frame to which zero data are added are subjected to fast Fourier
transform (FFT) and thereby converted into data in the frequency domain,
that is, a noise suppression spectrum X.sub.1(k) (S4). On the other hand,
every time the data of a noise estimation frame is extracted (i.e., for
each time interval corresponding to M/2 samples of the observation
signal), the data of the noise estimation frame is subjected to fast
Fourier transform and thereby converted into a signal in the frequency
domain, that is, a noise estimation spectrum X.sub.2(k) (S5). Every time
a noise estimation spectrum X.sub.2(k) is calculated (i.e., for each time
interval corresponding to M/2 samples of the observation signal), the
noise estimation spectrum X.sub.2(k) is subjected to proper dip
elimination processing or smoothing processing (S6). Every time the dip
elimination processing or smoothing processing is performed (i.e., for
each time interval corresponding to M/2 samples of the observation
signal), an operation of estimating a current noise spectrum N(k) is
performed on the basis of a noise estimation spectrum X.sub.2'(k)
produced by the dip elimination processing or smoothing processing and
estimation values of a preceding noise spectrum (S7).

[0041]Every time a noise suppression spectrum X.sub.1(k) and a noise
spectrum N(k) are calculated (i.e., for each time interval corresponding
to M/2 samples of the observation signal), the noise spectrum N(k) is
subtracted from the noise suppression spectrum X.sub.1(k), whereby a
noise-suppressed sound spectrum G(k) is calculated (S8). The sound
spectrum G(k) is subjected to inverse fast Fourier transform (I-FFT) and
thereby converted into a signal in the time domain, that is, an audio
signal (S9). Audio signals of frames that are obtained at the time
intervals of M/2 samples of the observation signal are connected to each
other (S10) and output as a continuous audio signal g(n), which will be
output as a sound from a speaker device, used for speech recognition
processing for the speaker, or used for some other purpose.

[0042]In FIG. 2, "(b) Process after noise suppression" is step S10 (frame
combining). More specifically, (N-M) tail samples corresponding to the
added zero data are removed from the frame of N samples obtained by the
inverse fast Fourier transform (S9), whereby a frame is obtained which
has M samples as in the original state. The data of each of frames of M
samples that are obtained at the time intervals of M/2 samples of the
observation signal is multiplied by a triangular window (i.e., the data
are given a gain characteristic that increases linearly from 0 to 1 in
the first half frame of the one frame length (the time length of M
samples) and decreases 1 to 0 In the second half frame). Resulting frames
are added to each other with an overlap of a 1/2 frame, whereby a
continuous audio signal is generated. As a result, a continuous audio
signal is obtained which is free of disconnections or steps between the
frames.

[0043]Next, an embodiment of a noise suppressing apparatus for executing
the above-described noise suppressing process of FIG. 1 will be
described. This embodiment is directed to a case that the following
settings are made:

[0047]FIG. 3 shows functional blocks of the noise suppressing apparatus.
An input signal (audio signal with noise) x.sub.0(n) is input to both of
a noise spectrum output section 10 and a noise suppressing section 12.
The audio signal with noise that is input to the noise spectrum output
section 10 is first subjected to a frequency analysis for noise
estimation in a noise estimation spectrum analyzing section 14. More
specifically, every time an input signal of M/2 samples (256 samples) is
newly input, a frame extracting section 16 extracts an input signal of
latest N (4,096) samples. A fast Fourier transform section 18 performs
fast Fourier transform on the extracted frame and thereby converts it
into data in the frequency domain, that is, spectrum data (discrete
Fourier transform data) X.sub.2(k) (k=0, 1, 2, . . . ). An amplitude
spectrum calculating section 20 calculates an amplitude spectrum from the
calculated spectrum data X.sub.2(k).

[0048]A dip eliminating section 22 eliminates dips in the frequency
characteristic from the calculated amplitude spectrum. For example, the
dip elimination processing is performed in the following manner. First,
the amplitude spectrum is subjected to smoothing processing in a
smoothing processing section 24. For example, the algorithm of the
smoothing processing may be a moving average method, in which an
amplitude value at the center of a prescribed number of consecutive
frequency points (i.e., a prescribed frequency band) is replaced by an
average of amplitude values at these frequency points. If the number of
consecutive frequency points used in one averaging operation (i.e., the
frequency bandwidth in which to calculate an average value) is set at
eight, for example, the substantial frequency resolution of a smoothed
amplitude spectrum (noise estimation amplitude spectrum) becomes equal to
that of a noise suppression amplitude spectrum. The average calculation
and the amplitude value replacement are performed while the frequency
point is shifted by one point each time, whereby an amplitude spectrum is
calculated that is smoothed over the entire frequency band.

[0049]Instead of the moving average method, a moving median method may be
employed as an algorithm of the smoothing processing of the smoothing
processing section 24. In the moving median method, an amplitude value at
the center of a prescribed number of (e.g., eight) consecutive frequency
points (i.e., a prescribed frequency band) is replaced by a median of
amplitude values at these frequency points. The extraction of a median
amplitude value and the amplitude value replacement are performed while
the frequency point is shifted by one point each time, whereby an
amplitude spectrum is calculated that is smoothed over the entire
frequency band.

[0050]In the dip eliminating section 22, a comparing section 26 compares
the amplitude spectrum that has been smoothed by the smoothing processing
section 24 with the unsmoothed amplitude spectrum and thereby chooses
larger values at respective frequency points. The comparing section 26
thus outputs, as a noise estimation amplitude spectrum |X.sub.2(k)|, a
continuous characteristic that is a connection of the chosen values. A
dip-eliminated noise estimation amplitude spectrum |X.sub.2(k)| is thus
obtained.

[0051]FIG. 4 shows the operation of the dip eliminating section 22 (only
part (frequency range: 1 to 100 Hz) of the entire amplitude spectrum is
shown in an enlarged manner). An unsmoothed amplitude spectrum A and an
amplitude spectrum B that has been smoothed by the moving average method
are compared with each other and larger values (indicated by dots) are
chosen at respective frequency points. And a continuous characteristic
that is a connection of the chosen values is output from the dip
eliminating section 22 as a dip-eliminated amplitude spectrum. As a
result, dips (valleys) are removed from the amplitude spectrum A and
processing noise is reduced.

[0052]Alternatively, the comparing section 26 shown in FIG. 3 may be
omitted (i.e., only the smoothing processing section 24 is provided in
place of the dip-eliminating section 22). In this case, an output signal
of the smoothing processing section 24 (i.e., an amplitude spectrum that
has been smoothed by the moving average method, the moving median method,
or the like) is output from the noise estimation spectrum analyzing
section 14 as a noise estimation amplitude spectrum |X.sub.2(k)|.

[0053]Referring to FIG. 3, the noise estimating section 28
estimation-calculates an amplitude spectrum of noise included in the
observation signal (hereinafter referred to as "noise amplitude
spectrum") according to an arbitrary estimation algorithm on the basis of
the dip-eliminated or smoothed amplitude spectrum. The dip eliminating
section 22 (or the smoothing processing section 24 that replaces the dip
eliminating section 22) may be disposed downstream of the noise
estimating section 28 rather than upstream of it.

[0054]On the other hand, in a suppression spectrum analyzing section 30,
the input signal (audio signal with noise) x.sub.0(n) that is input to
the noise suppressing section 12 is first subjected to a frequency
analysis for noise suppression (i.e., for generation of an observation
signal spectrum as a minuend from which to subtract a noise spectrum).
More specifically, every time an input signal of M/2 samples (256
samples) is newly input, a frame extracting section 32 extracts an input
signal of latest M (512) samples. A zero data generating section 34
generates zero data of (N-M) samples (3,584 samples). An adding section
36 adds the zero data of (N-M) samples after the end of the input signal
of M samples that has been extracted by the frame extracting section 32,
and thereby equalizes the length of the extracted input signal to the
noise estimation frame length T2 formally. A fast Fourier transform
section 38 performs fast Fourier transform on the zero-data-added data
and thereby converts the data into data in the frequency domain, that is,
spectrum data (discrete Fourier transform data) X.sub.1(k) (k=0, 1, 2, .
. . ), which are output as a noise suppression spectrum.

[0055]A suppression calculating section 40 performs noise suppression
processing according to an arbitrary suppression algorithm on the basis
of the noise suppression spectrum X.sub.1(k) that is output from the
suppression spectrum analyzing section 30 and the noise amplitude
spectrum |N(k)| that is output from the noise spectrum output section 10.
A noise-suppressed sound spectrum G(k) that is output from the
suppression calculating section 40 is subjected to inverse fast Fourier
transform in an inverse fast Fourier transform section 42 and thereby
returned to a signal in the time domain. Since the signal that is output
from the inverse fast Fourier transform section 42 is data of N (4,096)
samples, the lower (N-M) samples (3,584 samples) corresponding to the
zero data are removed from the signal by an output combining section 44,
whereby data of M (512) samples (i.e., samples of the original number)
are obtained. Frames are connected to each other, whereby a continuous
audio signal g(n) is output.

[0056]FIG. 5 shows specific examples of the noise estimating section 28
and the suppression calculating section 40. In the noise estimating
section 28, a spectrum envelope extracting section 45 extracts an
envelope |X.sub.2'(k)| of the noise estimation amplitude spectrum
|X.sub.2(k)| that is output from the noise estimation spectrum analyzing
section 14 shown in FIG. 3 by eliminating fine peak/valley
characteristics included in the noise estimation amplitude spectrum
|X.sub.2(k)|, for the following reason. If the noise estimation amplitude
spectrum |X.sub.2(k)| itself is used in calculating a correlation value
(described later), the spectrum correlation value becomes too small and
discrimination between sound intervals and noise intervals becomes
unclear. It is expected that an average spectrum of noise has a smooth
distribution that is almost uniform over a wide band if the average
spectrum is obtained by repeating observations for a long time. However,
in a short period, a spectrum of noise has a variation (peaks and
valleys). On the other hand, in contrast to noise, a frequency
characteristic of a sound has large amplitude values in particular
frequency bands and is not uniform over the entire frequency band. In
this specific example, a noise spectrum is estimated by discriminating
noise that is distributed uniformly over the entire frequency band and a
sound having large amplitude values in particular frequency bands using
the magnitude of a spectrum correlation value. Therefore, fine
peak/valley characteristics of the noise amplitude spectrum are
eliminated.

[0057]For example, the spectrum envelope extracting section 45 extracts an
envelope by performing lowpass filter processing on the noise estimation
amplitude spectrum |X.sub.2(k)| which is regarded as a time waveform. For
example, the lowpass filter processing may be such that the noise
estimation amplitude spectrum |X.sub.2(k)| is directly input to a lowpass
filter or is subjected to moving average processing in the frequency axis
direction. Another method for extracting an envelope |X.sub.2'(k)| of the
noise estimation amplitude spectrum |X.sub.2(k)| by the spectrum envelope
extracting section 45 is such that the noise estimation amplitude
spectrum |X.sub.2(k)| is further subjected to Fourier transform (cepstrum
analysis).

[0058]A noise amplitude spectrum initial value output section 46 outputs
initial values of a noise amplitude spectrum. That is, initial values are
set because immediately after activation of this apparatus there are no
noise amplitude spectrum data to be referred to. Examples of the method
for setting noise amplitude spectrum initial values are as follows:

[0059](Method 1) Data of only background noise (i.e., mixed with no
sound), which are input immediately after activation, are subjected to
Fourier transform, and amplitude spectrum data calculated from
Fourier-transformed data are set as noise amplitude spectrum initial
values.

[0060](Method 2) Amplitude spectrum data corresponding to background noise
are held in a memory in advance, and read out and set as noise amplitude
spectrum initial values at the time of activation. Alternatively,
envelope data of amplitude spectrum data corresponding to background
noise are held in a memory in advance, and read out and set as initial
values of noise amplitude spectrum envelope data at the time of
activation.

[0063]A correlation value calculating section 54 calculates a correlation
value (correlation coefficient) .rho. of the noise estimation amplitude
spectrum envelope |X.sub.2'(k)| of the current frame that has been
extracted by the spectrum envelope extracting section 45 and the noise
amplitude spectrum envelope |N.sub.0'(k)| that has been extracted by the
spectrum envelope extracting section 52. With the noise estimation
amplitude spectrum envelope |X.sub.2'(k)| and the noise amplitude
spectrum envelope |N.sub.0'(k)| written as

|X.sub.2'(k)|=x.sub.k (k=1,2, . . . K); and

|N.sub.0'(k)|=y.sub.k (k=1 2, . . . K),

the correlation value .rho. is calculated according to the following
Equation (1):

[0064]The noise amplitude spectrum calculating section 50 calculates a
noise amplitude spectrum |N(k)| for the audio signal in the signal
interval of the current observation according to the following Equation
(2) using the calculated correlation value .rho.:

[0065]|N(k)|: the noise amplitude spectrum that is estimated for the audio
signal of the frame being observed;

[0066]|N.sub.0(k)|: the noise amplitude spectrum that was estimated for
the audio signal of the frame that was observed last time (a half frame
earlier);

[0067]|X.sub.2(k)|: the noise estimation amplitude spectrum of the frame
being observed;

[0068]: the correlation value of the envelope of the audio signal spectrum
of the frame being observed and the envelope of the noise spectrum that
was estimated for the audio signal of the frame that was observed last
time; and

[0069]l and m: constants (l.gtoreq.1, m.gtoreq.0).

[0070]Equation (2) is to estimate a new noise amplitude spectrum |N(k)| by
adding together the noise amplitude spectrum |N.sub.0(k)| estimated last
time (a half frame (T1/2) earlier) and the noise estimation amplitude
spectrum |X.sub.2(k)| calculated this time at a ratio that depends on the
calculated correlation value .rho.. More specifically, when the
correlation value .rho. is small, it is judged that the sound component
is dominant in the input signal (i.e., a sound-existing interval).
Therefore, addition is made in such a manner that the proportion of the
noise amplitude spectrum |N.sub.0(k)| estimated last time is set high and
that of the noise estimation amplitude spectrum |X.sub.2(k)| calculated
this time is set low. That is, the noise amplitude spectrum |N(k)| is
prevented from varying being influenced by the sound component. In
contrast, when the correlation value .rho. is large, it is judged that
the sound component is a minor part of the input signal (i.e., a silent
interval). Therefore, addition is made in such a manner that the
proportion of the noise amplitude spectrum |N.sub.0(k)| estimated last
time is set low and that of the noise estimation amplitude spectrum
|X.sub.2(k)| calculated this time is set high. That is, the noise
amplitude spectrum |N(k)| is caused to vary so as to follow a gentle
variation of stationary noise. When the correlation value .rho. is
infinitely close to 1, the noise amplitude spectrum |N.sub.0(k)|
estimated last time and the noise estimation amplitude spectrum
|X.sub.2(k)| calculated this time are added together at an even ratio
(0.5:0.5). In this manner, the noise amplitude spectrum is updated mainly
in silent intervals.

[0071]In Equation (2), the parameter l is a constant for adjusting the
sensitivity to a small correlation value. The degree of updating of noise
amplitude spectrum estimation values of low correlation becomes smaller
as the l-value increases. In Equation (2), the parameter m is a constant
for adjusting the degree of updating. The degree of updating decreases as
the m-value increases.

[0072]In the suppression calculating section 40, the noise suppression
spectrum X.sub.1(k) is input to an amplitude spectrum calculating section
56 and a phase spectrum calculating section 58. The amplitude spectrum
calculating section 56 calculates an amplitude spectrum |X.sub.1(k)| of
the noise suppression spectrum X.sub.1(k) according to the following
Equation (3):

|X.sub.1(k)|={X.sub.R(k).sup.2+X.sub.l(k).sup.2}.sup.1/2 (3)

where

[0073]X.sub.R(k): the real part of X.sub.1(k); and

[0074]X.sub.1(k): the imaginary part of X.sub.1(k).

[0075]The phase spectrum calculating section 58 calculates a phase
spectrum .theta.(k) of the noise suppression spectrum X.sub.1(k)
according to the following Equation (4):

(k)=tan.sup.-1{X.sub.1(k)/X.sub.R(k)} (4)

[0076]A spectrum subtracting section 60 calculates a
noise-amplitude-spectrum-eliminated amplitude spectrum |Y(k)| of the
audio signal of the current frame by subtracting the noise amplitude
spectrum |N(k)| of the current frame calculated by the noise estimating
section 28 from the noise suppression amplitude spectrum |X.sub.1(k)| of
the current frame calculated by the amplitude spectrum calculating
section 56 according to the following Equation (5):

|Y(k)|=|X.sub.1(k)|-|N(k)| (5)

[0077]If |X.sub.1(k)|-|N(k)| becomes negative at certain frequency points,
it means over-subtraction. It is preferable that the difference |Y(k)|
being a negative value not be kept as it is but be changed to 0.

[0078]A recombining section 62 recombines the amplitude spectrum |Y(k)| of
the audio signal of the current frame that has been calculated by the
spectrum subtracting section 60 and the phase spectrum .theta.(k) of the
noise suppression spectrum X.sub.1(k) of the current frame that has been
calculated by the phase spectrum calculating section 58 and thereby
generates a complex spectrum given by the following Equation (6), that
is, a noise-suppressed sound spectrum G(k):

[0080]FIG. 6 shows output waveforms that were obtained when stationary
noise was input to noise suppressing apparatus. Symbol (a) denotes
original noise. Symbols (b) and (c) denote noise-suppressed outputs of a
conventional spectrum subtraction method in which the length of frames
extracted from an observation signal was common to the purposes of noise
estimation and noise suppression. The output (b) corresponds to a case
that the extracting frame length was set at 32 ms, and the output (c)
corresponds to a case that the extracting frame length was set at 256 ms.
Symbols (d) and (e) denote noise-suppressed outputs of the noise
suppressing method according to the invention in which the extracting
frame length for noise estimation (T2) and that for noise suppression
(T1) were set at 256 ms and 32 ms, respectively. The output (d)
corresponds to a case that the dip elimination processing of the dip
eliminating section 22 (see FIG. 3) was not performed, and the output (c)
corresponds to a case that the dip elimination processing was performed.
As shown in FIG. 6, degrees of attenuation from the original noise (a)
were

[0085]It is therefore concluded that the spectrum subtraction methods
according to the invention of (d) and (e) provide greater noise
suppression effects than the conventional spectrum subtraction methods of
(b) and (c). Of the spectrum subtraction methods according to the
invention, the noise suppression effect is greater in the case of (e)
where the dip elimination processing is performed than in the case of (d)
where the dip elimination processing is not performed.

[0086]FIG. 7 is a waveform diagram of a case that a sound with noise is
input to the noise suppressing apparatus according to the invention. In
this case, the noise estimation frame length T2 is set at 256 ms and the
noise suppression frame length T1 is set at 32 ms. Symbol (a) denotes a
sound with noise. Symbol (b) denotes a noise-suppressed output. And
symbol (c) denotes suppressed (eliminated) noise. It is seen from FIG. 7
that the sound (b) is obtained by suppressing the stationary noise (c) in
the sound (a) with noise.

[0087]The above embodiments employ the amplitude spectrum subtraction
method in which a noise amplitude spectrum |N(k)| is estimated on the
basis of an envelope |X.sub.2'(k)| of an amplitude spectrum |X.sub.2(k)|
of an input signal and noise suppression is performed by subtracting the
noise amplitude spectrum |N(k)| from an amplitude spectrum |X.sub.1(k)|
of the input signal. Alternatively, a power spectrum subtraction method
may be employed in which a noise power spectrum |N(k)|.sup.2 is estimated
on the basis of an envelope |X.sub.2'(k)|.sup.2 of a power spectrum
|X.sub.2(k)|.sup.2 of an input signal and noise suppression is performed
by subtracting the noise power spectrum |N(k)|.sup.2 from a power
spectrum |X.sub.2(k)|.sup.2 of the input signal.

[0088]Although in the above embodiments the noise estimation processing is
necessarily performed every prescribed time interval (every time T1/2
elapses), it may be performed every time a proper occasion arises. For
example, a process may be employed in which intervals in which noise
estimation can be performed easily such as silent intervals or faint
sound intervals are detected in real time and the noise estimation
processing is performed only in those intervals (i.e., the noise
estimation processing is not performed (i.e., it is suspended) in the
other intervals). The noise estimation processing may be suspended in
intervals with a small noise variation or intervals in which reduction in
processing load is desired. In these cases, in intervals in which the
noise estimation processing is suspended, a process may be employed in
which the data (noise amplitude spectrum |N.sub.0(K)|) are not updated in
the noise amplitude spectrum updating section 48 and the noise
suppression processing is performed on the basis of a latest (i.e.,
immediately before the suspension) noise amplitude spectrum |N.sub.0(k)|
held by the noise amplitude spectrum updating section 48.

[0089]Although the above embodiments are directed to the case of using FFT
as a frequency analyzing method, the invention may employ frequency
analyzing methods other than FFT.

[0090]In the above embodiments, the time window length in which to extract
an observation signal for noise suppression (i.e., the noise suppression
frame length T1, the period of M samples) is set longer than the cutting
time interval (i.e., the period of M/2 samples) because overlap
processing is performed in the output combining. The above two kinds of
time intervals may be set identical if overlap processing is not
performed.

[0091]Although the invention has been described above in detail in the
form of the particular embodiments, it is apparent to those skilled in
the art that various changes and modifications are possible without
departing from the spirit, scope, or the range of intent of the
invention.

[0092]The invention is based on the Japanese Patent application No.
2005-144744 filed on May 17, 2005, the disclosure of which is
incorporated by reference herein.